NIMRS-21Pin Decoder

NIMRS-21Pin-Decoder

A miniature DCC decoder for model railways featuring an ESP32-S3 microcontroller, designed to fit standard 21-pin sockets. Includes motor control, sound output, and lighting control in a 30x15.5mm footprint (following NMRA spec).

I don't quite have the time to work on this project at the momemnt but Kyle (ProjectInitiative) is starting to work on firmware for this board. You can follow the progress here: https://github.com/ProjectInitiative/NIMRS-Firmware

Features

• Compact Design: 30mm × 15.5mm × 1.0mm 4-layer PCB
• Power Management: Dual TPS63070 buck-boost converters for stable 9V (motor) and 3.3V (logic) supplies
• Quiet Operation: DRV8213 motor driver with real-time adjustable off-time current regulation
• Audio System: MAX98357A I2S amplifier driving 4-8Ω speaker
• Wireless Capable: U.FL antenna connector for ESP32-S3's Wi-Fi
• DCC Compatibility: Handles ~15V square wave AC input

Technical Specifications

Core Components

• MCU: ESP32-S3FN8 (240MHz dual-core, 8MB embedded flash)
• Motor Driver: TI DRV8213
• Audio Amplifier: MAX98357A
• Power Regulation: Dual TPS63070 buck-boost converters

PCB Stackup

1. Top Layer: Signals with GND pour
2. Inner 1: Full GND plane
3. Inner 2: Signals with 3V3 pour
4. Bottom Layer: GND plane with sparse signals

Design Highlights

• Single-sided assembly and 0.3/0.4mm for cost efficiency
• Optimized ground planes for EMI reduction
• Decoupling buck-boost for stable motor PWM operation
• WiFi + BT support with U.FL antenna

Getting Started

1. Clone the repository:

   git clone https://github.com/CDFER/NIMRS-21Pin-Decoder.git

2. Open in KiCad v9.0+

3. Review design files:

   • Interactive PCB Viewer
   • Schematic
   • PCB Layout

Repository Structure

├── Custom_Footprints.pretty  # Custom component footprints
├── Datasheets               # Component technical documents
├── Images                   # PCB visuals and renders
├── LICENSE
└── README.md

For those wondering the 21-Pin decoder socket uses a 1.27mm, 22Pin back entry connector and I am using the BD050-22-A-0-0500-L-D from GCT (the CAD file, footprint and symbol can be found in the custom footprints library I have made).

License

MIT License - See LICENSE for details.

Contributing

Pull requests welcome! Please discuss major changes via issues first.